Examination of compound formation at interface of tin–bismuth–silver solder and copper substrate by using electron probe micro analysis

The wettability of tin–bismuth–silver solders on copper was studied by using energy‐dispersive x‐ray spectrometry (EDXRS) for elemental analysis. Four kinds of fluxes were used. The wettability of conventional eutectic tin–lead solder was much higher than that of solders based on tin–bismuth systems. The poor wettability of tin–bismuth solders may be caused by the growth of dendritic copper–tin compounds at the copper/solder interface, in contrast to the case of tin–lead solder. The wettability was improved by addition of silver to the solder. In the tin–bismuth–silver system, EDXRS mapping of elements revealed that silver was abnormally concentrated at the copper/solder interface even in the initial stage of their contact. When using FR‐type fluxes, the solder exhibited maximum wettability in the range from 1–4 mass% Ag addition. The layer thickness of the Cu–Sn compound formed on Cu was smallest with a 2% Ag addition. This may correspond to the improvement of the solder wettability. Copyright © 2002 John Wiley & Sons, Ltd.